How to breed lots of Roostros

Rad Slimes can be found in The Indigo Quarry.

They were added to the game on April 13, 2016 with the aptly named Indigo Quarry Update (v0.3.0)*.

Rad Slimes are Slimes found exclusively in The Indigo Quarry. Unlike other Harmful Slimes they do not deal damage directly, but when exposed to their aura for long enough Beatrix will start taking constant damage as long as she remains within the aura.

^{Excerpt and photo from Slime Rancher Wikia}

_{*Rad Slimes were obtainable in earlier versions of the game, but were unobtainable for versions between v0.2.0 and v0.3.0.}

It turns out there is an advantage to, at the very least, Carrots and Pogofruits. When grown in a garden, food like Cuberries and Mint Mangoes will only produce up to 15 of each in a single harvest, while Carrots and Pogofruits will produce up to 20.
This is particularly useful when feeding Fire Slimes, as any kind of food can be incinerated to produce ash.

While this makes me think a similar mechanic is in effect for Hen Hens, maybe with them breeding or growing up faster than their specialized counterparts, that is harder to test due to the random nature of egglaying.